A number of techniques exist for preparing such wafers. One example of one of the most effective current techniques for fabricating sSOI or SGOI type wafers is the production of an active layer of strained silicon (sSi) or relaxed SiGe by transfer thereof onto an insulating support (for example a layer of SiO2 on a silicon substrate) using the SMARTCUT® technique to produce the desired heterostructure. Particular examples of implementations of such fabrication techniques are described in United States patent application US 2004/053477 and International patent application WO 2004/006311.
The finishing step of such wafers involves a selective etching procedure to eliminate the SiGe layer subsisting above the silicon layer after transfer thereof to the “receiver” substrate and detachment from the “donor” substrate. Selective etching is a chemical attack method which can selectively eliminate the upper layer of SiGe without attacking the next layer of strained silicon, by adjusting the composition of the chemical solution, generally a CH3COOH/H2O2/HF mixture.
In a second step, a cleaning step is carried out which consists of treating the wafers with cleaning solutions.
The known standard method, termed “RCA”, for cleaning the surfaces of wafers comprising a surface layer of semiconductor material comprises:                a first step of cleaning using an SC1 (standard clean 1) solution (or APM, ammonium hydroxide peroxide mixture); containing ammonium hydroxide or ammoniacal solution (NH4OH), hydrogen peroxide (H2O2), and deionized water (H2O);        a second cleaning step using an SC2 (standard clean 2) solution (or HPM, hydrochloric peroxide mixture); containing hydrochloric acid (HCl), hydrogen peroxide (H2O2) and deionized water.        
The first solution, SC1, generally used at between 50° C. and 80° C., is principally intended to withdraw particles isolated on the surface of the wafer and particles buried near the surface, and to render the surface hydrophilic.
The second solution, SC2, generally used at between 70° C. and 90° C., is principally intended to remove metallic contaminants which may have been deposited on the surface of the wafer, principally by forming chlorides.
One of the disadvantages of RCA cleaning arises from using SC1 and SC2 solutions. The presence of ammoniacal solution means that a certain amount of etching cannot be avoided: the layer of strained silicon which has been released by selective etching of the SiGe layer is then etched by the RCA cleaning solution. In general, the roughness after a standard RCA clean is of the order of 1 to 2 angstroms (Å) root mean square (rms).
When producing sSOI products, the layer of strained silicon denuded by etching is the useful layer of the final product and so constraints in terms of final roughness and thickness are tight. It will thus be understood that a post selective etching cleaning step must satisfy requirements regarding effective removal of particles without being too aggressive, so as not to deteriorate the roughness and thickness of the final layer. In general, the thickness etched by the SC1 solution may be as high as 10 Å for a 200 Å strained silicon layer, knowing that RCA cleaning etches strained silicon at a rate of about 10 angstroms per minute (Å/min).